1. Field of the Invention
This invention relates in general to the field of permanent magnet devices. More particularly, the invention relates to magnetic bottles that function as magnetostatic traps for charged particles.
2. Description of the Prior Art
Magnetostatic traps of the magnetic-bottle variety have been used to confine ions and other charged particles that are too hot or too cold to be permitted to interact with the walls of a container vessel, or that would react violently with them. For example, some magnetic bottles hold dense hot plasmas of isotopes for nuclear interactions. Other magnetic bottles store isolated ion or atomic systems at extremely low temperatures.
The operation of magnetic bottles is based on the principle that a charged particle with velocity perpendicular to the bottle's magnetic field lines travels in a circle, whereas a particle moving parallel to the field is unaffected by it. In general, such particles have velocity components both parallel and perpendicular to the field lines and, therefore, move in helical spirals.
One well known type of magnetic bottle confines particles therein through the use of magnetic mirrors, i.e., regions at opposite ends of the bottle where magnetic fields increase abruptly in strength. As a particle approaches these ends, the concentrated magnetic fields cause it to spiral into ever-tightening helixes. Essentially, these concentrated fields act as magnetic mirrors by reflecting the charged particles back into the central region of the bottle where the process is repeated. More specifically, the time-averaged circular motion of a confined particle effectively acts as a current loop with an associated magnetic moment that the gradient of the magnetic bottle field repels.
In prior art apparatus, the size of a magnetic bottle usually varies inversely with the strength of its magnetic field. In general, the greater the strength of the magnetic field, the smaller the region in which the particles can be trapped. Those concerned with the development of charged particle devices, such as plasma gas discharge tubes, plasma display devices and free-electron lasers, have recognized the need for improved compact magnetic bottles capable of confining particles in relatively small regions. Such compact magnetic bottles are also especially suitable for operation in confined spaces where weight and size pose serious problems, such as in aircraft, submarines, missiles and ballistic devices.
One of the most critical problems confronting designers of compact magnetic bottles has been the fabrication of permanent magnets capable of generating gradient magnetic fields of high intensity in a structure having a minimum of mass and bulk. Ideally, such compact magnetic structures must also be inexpensive to manufacture, and be sufficiently stable to operate reliably under adverse conditions such as high temperatures and accelerations.